One of the largest problems in locating people or devices within a building is the precise location in a building where GPS coverage is not available. GPS essentially stops working at the door of a building such that inside the building one does not have the ability to access satellites. This is true even for assisted GPS systems depending on how far into a building one goes. It is understood that at least at some points in a building GPS signals are too attenuated to be usable. Therefore one can not get a precise in-building location utilizing the GPS system.
While triangulation be utilized from the cell towers, one can arguably get within 100 feet, however 100 feet is clearly insufficient and what is required is plus or minus two feet to properly locate devices or human beings within a building.